106 



THE ELASMOBRANCH FISHES 



branchial cartilages. The shorter or inner fibers join the eeratobranchial carti- 

 lage, and the outer fibers in the sharks pass into the dorsal interbranchial 

 fibers. 



In the rays the interbranchials are somewhat like those of the sharks. Here, 

 however, the dorsal and ventral fibers do not form a continuous muscle even 

 at the outer margin. The fibers both dorsally and ventrally arise as in the 

 sharks. The inner dorsal fibers are inserted also on the epibranchial {eh., fig. 



A B 



Fig. 108. Interbranchial musculature. (From Marion.) A. Acanthias. B. 2?«m erinacea. 



ad., adductor muscle; br., branchial ray; ch., ceratoliranchial cartilage; eh., epibranchial 

 cartilage; ex.b., extrabranchial cartilage; td., longitudinal tendon. 



108b) and the inner ventral fibers on the eeratobranchial cartilages (ch.). The 

 outer fibers are attached to the cartilaginous branchial ray which proceeds 

 outward at the angle between the epibranchial and eeratobranchial segments. 

 Some of the most median fibers ventrally take origin from the fascia of the 

 coracomandibularis muscle. 



INTEEARCUALES 



The interarcuales, as in Heptanchus, are divided into two systems, a dorsal 

 (medial of Marion) and a lateral system. The subspinalis, which is considered 

 li}^ some as the first of the dorsal systems, arises from the base of the cranium, 

 the ventral side of the vertebrae, and the fascia ventral to the longitudinal 

 bundle, and tapers back to be inserted near the tip and laterally usually on the 

 first pharyngobranchial cartilage. In Scyllium, as in Heptanchus, the sub- 

 spinalis is divided into two bands; in Sqtiatina it is very slender; and in Sci/m- 

 nus and in the rays it is generally absent. 



The true dorsal interarcuales {ia.d., fig. 109) unite succeeding pharyngo- 

 branchial segments. The dorsal interarcuales vary in number; five in Hep- 

 tanchus, four in Hexanchus, three in Acanthias and Heterodontus, and two 

 in Raja. In Scijmni(s the tips of the pharyngobranchial segments are bound by 

 strong ligaments to a sheet of connective tissue ventral to the spinal column. 



In Acanthias (fig. 109a), as in Heptanchus and many other sharks, the 

 lateral interarcuales system (ia.l.) consists of V-shaped muscles, one limb of 



